Is it acceptable to have the FRS as a stand along document without a URS document?

I'm validating (Computer System Validation) a vendor delivered GCLP system, and have been detailing the deliverables to be created in the Validation Plan. My understanding is that both the User Requirements Specification (URS) and Functional Requirements Specification (FRS) are created for a complex GCLP system, where the the FRS describes how the system will do the things listed by the business in the URS. However in this case, the Vendor is refusing to link the FRS and URS together, meaning I do not know which FRS section corresponds to which URS section. Is it acceptable to have the FRS as a stand along document without a URS document? At my previous company, the FRS and URS had been combined into a RS document, so that's an option as well in case we are not able to link FRS and URS together, it's better to have just one RS document. Let me know what you think?

Dear Aron,
as my point of you is not a best practice. but i do agree it's done in common practice. URS is intended to capture around a business process all regulatory and criticla business needs to perform their business without focus on "a specific system" where the FS comes with a system solution to address some of those needs whereas other URS will be address by process or manual mitigation controls. if we mix both there is a risk to omit some requirements from process view and to focus on operating software capabilities.

The short answer is yes, it is acceptable to have separate documents. However, this is where the use of a Traceability Matrix may be useful. In my experience, the FRS is often (usually) a separate document from the URS, but they should, as you state, be in lockstep. A Traceability Matrix describes the links between all the various components within the requirements documents, ideally from specification through to (successful) testing.